Island ecology of arthropods : I. Diversity, niches, and resources on some Seychelles islands.

1. Twenty-five species of web-building spiders (Araneae) were studied by examining standard study areas on 5 islands of the Central Seychelles group that differ in size and degree of isolation. The number of individuals and of species on each island was correlated to the area, altitude, degree of isolation, and resource availability of the respective island. The effect of these factors on niche breadth and niche overlap was also examined. 2. In respect to resource availability the structural diversity of each study area was determined by means of the Shannon-Weaver formula. Environmental conditions were broken down into 4 categories according to cover, soil, water, vegetational strata, and number of plant species. In addition to this quantitative method different habitats were defined according to existing plant associations (Table 2). Both methods produced similar results. 3. As the islands increased in size, both the spectrum of resources and the number of habitats also increased. Islands of lasrger surface revealed a higher mean as well as a greater range of structural diversity (Fig. 2). Associated with the increased spectrum of resources there was a significant increase of spider species. 4. The number S of web-building spider species increases with the size A of the island according to the Wilson formula S=cA z (Fig. 3). The present study revealed the coefficient z to be remarkably low. 5. The degree of isolation of an island, calculated according to macArthur and Wilson (1967), did not explain the differing numbers of spider species on the different islands. 6. Niche breadth and niche overlap were calculated in terms of numbers of habitats and of ranges of structural diversity. The most abundant spider species had the greatest niche breadth. The average niche breadth became smaller with the increase of island size. Specific niche breadths of spiders on different islands did not vary significantly between islands (Table 6). But in all cases there was a reduced niche overlap in the more diverse spider communities (Tables 5 and 8). 7. The total density of the spider community was highest in the structurally most diversified habitats. The total density of web spiders diminished with the decreasing size of the island. The correlation between total density and number of species was significantly positive. 8. The low value of the z coefficient in the species-area relationship is discussed. The possibility of a low ecologic saturation on smaller islands is considered. Ways in which species diversity may increase are examined. Analysis of resource availability proved to be more efficient for the solution of ecologic problems than the simple examination of the species-area relationship.

Island ecology of arthropods : II. Niches and relative abundances of Seychelles ants (Formicidae) in different habitats.

1. The ant fauna of 3 islands of the Central Seychelles was studied by means of pitfall traps. The number of species and their abundances were investigated in relation to differences in island size, habitats, and microclimate conditions. In addition the effects of these factors on niche breadth and niche overlap were studied. 2. Nineteen and species were collected that belonged to 14 genera in 4 subfamilies (Table 2). Different numbers of species were found on different islands (Table 1). There are no significant correlations between the numbers of species, sizes of islands, and the available resources. The distribution of congeneric species agrees with the expected increase in 'relative diversity' on smaller islands (Fig. 3). 3. Species diversity and species distribution depend to some extent on habitat differences. This was evaluated by phytosociologic methods and microclimatic measurements. The composition of the ant fauna on different islands is more constant in undisturbed, autochthonous habitats than in those habitats that are strongly influenced by man. 4. Specific niche breadth and relative abundance of ant species showed a significantly positive correlation. Within comparable habitats, average niche breadth and niche overlap are greatest on the smallest island. Because the relative density also increases on this island, one cannot speak of a lower ecologic saturation compared with the larger islands.

Zur jagdstrategie einiger orientalicher Leptogenys-arten (Formicidae: Ponerinae).

The sociobiology of Leptogenys ocellifera especially its predatory behavior and food recruitment was investigated. Leptogenys binghami and Leptogenys diminuta were observed comparatively. L. ocellifera: Colony size, nesting sites, and nest emigration. L. ocellifera lives in large colonies of several tens of thousands workers. It nests in existing soil cavities. The nesting site is changed in irregular intervals. According to our results the nest emigration is released by environmental factors.Trail system. There are several elaborated permanent routes leading from the nest into the surroundings. Those routes can be used continuously day and night over a period of several months. Guard ants are posting along the trails. Mainly in the evening, males were observed on the trails running back and forth. This behavior is interpreted as mating running". In the peripheral surroundings of the nest the routes may change within few hours or even minutes according to the food supply.Recruitment trails. The workers leave the nest singly or in groups. When an ant meets a prey animal it returns to the closest frequented trail rubbing its gaster tip on the ground while the sting is intermittendly extruded. The alarm secretion is released from the venom gland. It works as both orientation and recruiting signal and attracts new workers from the trail. No tactile signals are involved in food recruitment. The new workers in their turn intesify the alarm trail. Prey carrying workers lay trails to the nest which recruit further ants. Once the food source is exploited the alarm effect, even on strong trails, would decrease within half an hour. Recruitment trails which have been used over some time may become permanent routes. Along the permanent routes in addition to the recruiting substance an orientation component could be detected which lasts at least over 5 hrs. Its source is still unknownPredatory behavior. During foraging the workers frequently show digging behavior especially on places with fresh soil. Alarm secretion increases the digging activity. By this method L. ocellifera captures mainly animals which live in the soil, especially termites and earth worms. Other arthropods and snails are also predated. Larger prey objects are hunted and cut into pieces cooperatively. The pieces are transported into the nest by single workers. Killed termites are layed down near or on the trails by the foragers. Such depots are emptied by other workers. L. binghami and L. diminuta: L. binghami lives in natural earth cavities. It forages singly as a rule. Its main prey animals are termites. The trail secretion is mainly used during nest emigration. This is performed in a fast and well organized way. L. diminuta nests near the surface of the soil. It is a typical group prdator. The successful scouts lay recruiting trails while returning into the nest. The observed raids contained three to more than hundred workers led only sometimes by the scout. Prey objects (mainly larger arthropods) are hunted and retrieved cooperatively.